1. Field of the Invention
The present invention relates to an electro-rheological fluid, and more particularly to an electro-rheological fluid having high resistance to dielectric breakdown (hereinafter, "dielectric breakdown strength"), and methods of manufacturing and storing the same.
2. Description of the Related Art
An electro-rheological fluid is a fluid that can significantly and reversibly change its rheological characteristics with electrical control. The phenomenon of dramatic change of the apparent viscosity of a fluid through the application of an electric field has been known as the Winslow effect for a long time. The application of this effect to components for electrically controlling devices or parts, such as clutches, valves, engine mounts, actuators, and robot arms has been discussed.
As a result, many proposals have been made on particles or liquid mediums used as a dispersoid for the purpose of obtaining a fluid having a high electro-rheological effect and excellent reproducibility. Conventionally, when a high voltage of 2.5 to 3.5 kV/mm or more is applied to an electro-rheological fluid (hereinafter occasionally referred to as "ERF") under a flow, it is found that electrical discharge generates frequently such that the fluid cannot be actually used.
It has been discovered that an oil type medium, such as dimethylsilicone or fluorosilicone, and anhydrous particulates such as carbonaceous particulates which form this ERF have dielectric breakdown strengths of 6 kV/mm or more respectively. Accordingly, even if both the oil type medium and the conductive particles used are materials having high dielectric breakdown strengths, it is found that the dielectric breakdown strength of the obtained ERF does not reach a desired level.
When such an electro-rheological fluid is applied to dampers or clutches, a practical vibration controlling effect or the like cannot be obtained due to electrical discharge, and thereby reliability lowers.
An object of the present invention is to provide an electro-rheological fluid made of an oil type medium and fine particulates by mixing, which is highly reliable and in which dielectric breakdown such as generation of electrical discharge hardly occurs at all even if high voltage is applied to the ERF. Further, a second object of the present invention is to provide a method of manufacturing an electro-rheological fluid by which such an electro-rheological fluid is obtained through a simple process and a method of storing an electro-rheological fluid which can prevent reduction in performance of the fluid at the time of conveyance and storage.
As a result of assiduous studies, the inventors have found that, in a state in which air or air forming gas (nitrogen, oxygen, argon, or the like) is included in an ERF made of an oil type medium and fine particulates by mixing, electrical discharge generates when a high voltage of 3.5 kv/mm or more is applied under a flow, and that the above-described objects are achieved by preventing the generation of electrical discharge. The present invention has been thereby completed.